A time to digital converter (TDC) outputs a digital representation of the time of arrival for each incoming pulse of a signal. A TDC can be formed by chaining together a string of inverters. A start-pulse is propagated through the inverter chain and sampled with a stop pulse. The number of inverters through which the start pulse passes provides a digital measure of the time from start to stop. The resolution associated with this type of TDC is typically limited by the gate delay of the inverters which is highly dependent on current, voltage and temperature. Also, the linearity of the TDC is limited because of device mismatches and typically traded against speed and resolution. Relatively small inverter stages are needed to improve the resolution of the TDC because small inverters have reduced parasitic capacitance. However, relatively large inverter stages are needed to improve device mismatch and linearity. Digital correction techniques and statistical methods can be used to linearize the transfer function of the TDC, but the resolution remains limited by gate delay.
Another type of TDC is the Vernier delay line which utilizes the difference in delay between two delay lines. However, device mismatch has an even greater adverse impact on the linearity of Vernier-based TDCs. In addition, a very long delay line is needed to achieve sufficient dynamic range. Other types of TDCs use a ring oscillator which is switched on when the time period measurement starts and off when the time period measurement ends. Switching can be done by gating inverter cells. Switching the ring oscillator on and off in this way sets the internal nodes of the TDC to a high impedance state when turned off. Noise shaping occurs when the parasitic capacitances of the ring oscillator maintain their voltages during the high impedance off state. Ring oscillator-based TDCs can achieve relatively high resolution and reject transistor mismatch effects. However, the high impedance off state creates a large sensitivity to noise and leakage currents. For example, conventional ring oscillator-based TDCs suffer from high leakage currents which affect oscillator voltages during high impedance states. With process scaling, the leakage current worsens and becomes strongly dependent of temperature. In addition, noise currents are injected into the high impedance nodes which also affect the oscillator voltages. Furthermore, the high impedance node voltages may be adversely affected by charge injection during switching. Counting errors can occur in the high impedance state because of the problems mentioned above. Conventional ring oscillator-based TDCs are highly voltage dependent during periods of stopping and starting, decreasing the noise-shaping performance of the TDCs.